@igel
Maybe the browser you're using is better on Windows. Also is it a different PC, or are you dualbooting?
@igel
Hhmm... There are 2 things I'm thinking about that might cause this.
1) differences in how the 2 OSes handle media codecs. Maybe there's something about that that makes browsers perform differently between platforms, and you just noticed with Misskey because it's a media rich site.
2) (which seems more likely to me) It's not exactly news that GPU acceleration in browsers doesn't work that well (if at all) in Linux.
You could try making a Linux live USB, booting into it on the laptop, and seeing if Misskey works slower in that environment than Windows.